Energy Release Phenomena due to Thermal Interaction between Fluids in Liquid Metal Cooled Reactors

Liquid metal cooled reactors are considered one of the most promising nuclear reactor concepts in the frame of Generation IV systems to be commercialized in the next decades. The interest for liquid metal coolants, in particular sodium, lead and lead-bismuth eutectic (LBE), is based on their thermal and nuclear properties that allow at having systems able to safely remove heat without appreciable “softening” of the neutron energy spectrum. As a consequence, their adoption might provide a better exploiting of the fuel. In addition, liquid metal cooled systems are potentially capable of trasmuting Minor Actinides and long lived radionuclides, thus representing a possible contribution to the solution of nuclear waste problem. Furthermore, a huge experience has already gained in liquid metal technologies because both sodium and heavy liquid metals have been already used as coolant in fast breeder reactors. For future reactors cooled by molten metals one of the most important safety issues, which is also one of the major challenges, is the interaction between water and LBE or lead (Coolant-Coolant Interaction, CCI) during postulated steam generator tube failures and the expansion phase of bubbles in sodium cooled fast reactors, generated by the interaction of fuel with the sodium coolant (Fuel-Coolant Interaction, FCI) in case of Core Disruptive Accident (CDA). Both CCI and FCI represent a threat for the integrity of the reactor’s structure, even though their likelihood of occurrence is relatively small. In fact, during the interaction, a huge amount of heat is transferred from the material at higher temperature (heavy liquid metal or melted fuel) to the colder material (water or sodium) in a very short timescale. Two threatening phenomena can result from interaction. The first one is the formation of shock waves that can damage the inner structures of the reactor and, in the particular case of heavy liquid metal reactors, might cause a sort of “chain effect” damaging other steam generator tubes. The second one is due to the rapid heat transfer rate that allows the hotter material to solidify and vaporizes the colder material, too. The vapour formed moves upwards compressing the cover gas region and increasing the pressure (expansion phase), resulting in another threat for structure integrity. The aim of this study is addressed to analyse and simulate the phenomena involved in CCI and FCI, with particular attention to the evaluation of the energy released in such interactions, in order to have the possibility of estimating the potential loads and the resulting damage on reactor structures. A scientific cooperation between ENEA Brasimone Research Centre and University of Pisa has been set up, in the frame of IP-EUROTRANS and ELSY FP6 Projects, concerning the heavy liquid metal-water interaction. In particular, the activity of University of Pisa has been focused on the simulation by the SIMMER III code of the experiments performed at ENEA Brasimone with LIFUS 5 facility. This activity has been aimed at characterizing further the interaction between heavy liquid metal and water in order to understand which conditions might lead to serious consequences for reactors. The use of existing tools for the analysis of energy release and the development of new ones based on existing theoretical models constituted a further contribution provided by this study. This topic is crucial because of the lack of representative experimental data. Regarding the activity focused on sodium fast reactors, an experimental campaign called SGI, which was performed in 1994 in Forschungszentrum Karlsruhe (now KIT), have been chosen. This choice is motivated because the injection of a high pressure gas into a stagnant liquid pool is a characteristic phenomena taking place during the expansion phase of a CDA. SGI campaign has been simulated again with SIMMER III code, thus contributing to its further qualification by the numerous experimental data available, except for the energy release.